The present invention relates to a transmitter for a tire condition monitoring apparatus, more specifically, to a transmission antenna of a transmitter.
Wireless tire condition monitoring apparatuses that allow a driver in a vehicle passenger compartment to check the conditions of vehicle tires have been proposed. One such monitoring apparatus includes transmitters and a receiver. Each transmitter is attached to a vehicle wheel to be located in the corresponding tire, and the receiver is located in the body frame of the vehicle. Each transmitter detects the condition of the associated tire, such as the internal pressure and the internal temperature, and wirelessly transmits a signal including data that represents the detected tire condition as radio waves through a transmission antenna. The receiver receives the radio waves from the transmitter through a reception antenna, and displays the condition of the tire on a display located in the passenger compartment as necessary.
Each transmitter typically includes a circuit board on which electronic circuit is formed and a casing accommodating the circuit board. The electronic circuit includes electronic components mounted on the circuit board, such as a pressure sensor and a signal processing element (for example, refer to Japanese Laid-Open Patent Publication No. 2003-112506).
To enhance the transmission performance, it is preferable to use large transmission antennas with a maximized transmitting surface. However, the transmission antenna of the transmitter disclosed in the above prior art is typically a wire antenna that has relatively small transmitting surface and is attached to the front surface or the back surface of the circuit board. To increase the transmitting surface of the wire antenna, the length or the diameter of the antenna needs to be increased. This not only increases the size of the transmission antenna, but also increases the sizes of the circuit board and the casing. It is thus difficult to simultaneously reduce the size of the antenna and improve the transmission performance of the antenna.